Wavelength division multiplexing is a commonly used technique that allows the transport of multiple optical signals on an optical fiber. The ability to carry multiple signals on a single fiber allows that fiber to carry a tremendous amount of traffic, including data, voice, video, etc. By using wavelength division multiplexing it is possible to use a single fiber for multiple channels, as opposed to the costly process of installing additional fibers.
In wavelength division multiplexing (WDM), multiple channels are carried on different wavelengths within a specified optical band. It is advantageous to carry as much information as possible in this optical band. This is especially true for optical transmission systems which employ optical amplifiers to overcome the losses encountered during transmission. These amplifiers tend to have a fixed optical band in which they can perform amplification. Therefore, increasing the information carrying capacity of this optical band utilizes the amplifiers to their fullest extent.
The information capacity of an optical band can be increased by increasing the line-rate of the individual channels or decreasing the channel spacing in the optical band. The line-rate of the channel refers to the rate at which information is transmitted, e.g., the number of bits per second (B/s). An increased line rate causes the channel to occupy more optical spectrum. In contrast, decreasing the channel spacing decreases the amount of optical spectrum allotted to each channel. These competing effects can be described by the concept of spectral efficiency. Spectral efficiency is defined as the line-rate divided by the channel spacing and is usually expressed in units of bits per second per Hertz (B/s/Hz). For example, a system using a 109 bits per second (1 GB/s) line-rate and a 100 GHz channel spacing would have a spectral efficiency of 0.1 B/s/Hz. It should be obvious that the spectral efficiency is a measure of the information carrying capacity of an optical band in that doubling the line-rate or halving the channel spacing results in twice the spectral efficiency and equivalently a system with twice the information carrying capacity.
The multiplexing and demultiplexing of optical channels is typically accomplished with wavelength selective optical filters. A conventional WDM multiplexer (MUX) and demultiplexer (DeMUX) employs a number of such filters, each filter adapted for passing one specific wavelength. Such conventional schemes are, therefore, disadvantageous in that multiple filters are required, adding to cost and complexity.
For example, in the classical “all-filter” approach the MUX and DeMUX are provided through a cascade of wavelength selective filters. This tends to result in the solution with the least amount of loss. However, there is a penalty in that the filters must have small bandwidths, smaller than the channel spacing because of the requirement to provide isolation and a second transmission path for adjacent channels. This tends to distort both the transmitted and adjacent channels both in amplitude and phase. The channel spacing can be increased to mitigate this optical filtering penalty, but the introduction of such ‘dead-bands’ decreases the spectral efficiency of the system making it less cost-effective.
Alternatively, using a power splitter and power combiner approach, the MUX and DeMUX are provided with the lowest filtering penalty but the highest loss. The MUX may be entirely wavelength insensitive while the DeMUX must have a stage of wavelength selective filtering. However, the amplitude and phase distortion of these filters is limited to the transmitted channel as adjacent channels are taken care of by the stages of power splitting. This requires many stages of power splitters and combiners which tends to increase the loss. For high channel count systems, therefore, this method becomes undesirable.
Separating and combining wavelengths in high spectral efficiency systems requires optical components which have high peak transmission over a relatively large bandwidth when compared to the channel spacing and which provide good isolation between closely spaced wavelengths. Therefore a method is needed for providing a MUX and DeMUX structure with reasonable loss and low filter penalty.
The issue of modularity has been addressed in the past by grouping wavelengths together to create a smaller number of wavelength dependent modules than the actual number of individual wavelengths. In general, wavelength specific modules tend to increase the number of modules that need to be developed and manufactured (affecting cost and time-to-market), marketed (decreasing the volume of the individual modules thereby increasing cost) and kept in inventory (both the vendor and the customer) for field replacement. Therefore it is advantageous to develop a solution which reduces the number of different modules required.
Furthermore, There is a need in optical transmission systems for optical add-drop multiplexing (OADM) at sites in the systems where traffic already exists on the optical fiber from different points of origin and having-different destinations. At these sites, it is also required to have access to add and drop traffic locally in a flexible manner. It is advantageous to reuse the same MUX and DeMUX architecture for this purpose thereby reducing the total number of different modules which would otherwise need to be developed. In addition, the use of the same modules for MUX, DeMUX, and OADM ensures the compatibility of the channel plan used for each purpose.